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The Effect of Lockstitch Sewing Machine Processing Parameters on The End-Product Quality Part 2

Prof. Dr. Ibrahim Elhawary a, Marwa Issa b

Sewing Processing Parameters effect on the seam quality

There are several sewing conditions affecting parameters of seam quality as shown in Figure (5). This article discusses the main factors such as stitch density, sewing machine speed, needle size, pressure of pressure foot, feed dog, thread tension, needle plate and stitch regularity or balance.
1)  Stitch Density
It has been concluded that the long Stitches are less durable and considered to be of lower quality because they are subject to abrasion and likely to snag. Therefore, the greater the number of stitches/ inch in a seam, the greater the seam efficiency and strength taking into consideration the fabric type. A high stitch density also has potential to increase seam puckering due to structural jamming along the seam line. 4 Therefore, it was recommended that decrease of stitch length and use of fabric with greater bending stiffness and increased extensibility, make seams free of pucker. 5
Another research has studied the effect of stitch density on the seam strength, elongation and efficiency using cotton woven fabrics with different warp and weft threads count. It has been concluded that, an increasing trend was detected confirming that as the stitch density increases the seam strength follows the same manner as shown in Figure (6). 

1)            Sewing machine Speed & Needle Heat
Due to the high needle speed, heat generation takes place at the surface of the needle. Excessive heat in and around the needle surface may create a seam damage which is the cause of poor seam efficiency and appearance in a garment. 4
The figure (7 and 8) and Table (1) show that the needle temperature rises with longer time of sewing. Results were recorded for different types of sewing threads so that Polyester/ Polyester core spun sewn thread was recommended to be used as it has low hairiness comparing with Polyester, cotton and Polyester/ cotton yarns. 7
In order to reduce the generated heat a cooling system , whenever machine stop or running, has been invented. In addition, Threads are being manufactured with low friction properties that can resist high temperatures. Moreover, new needle designs and coatings have been developed, and many more fabric finishes have become available, all claiming to reduce needle heating and to improve sewability.

1)    Needle Size And Feed Timing:
There are various types of needle in terms of size, needle point and sewing machine type as per figure (9) and Table (2). The needle size refers to the needle diameter. There are different systems in the apparel industry for measuring the same. The metric system is the simplest and most widely used in the apparel industry. A higher the metric count represents a greater needle diameter. 5
Select the type of needle based on the textile construction (i.e knit vs woven), and its size is determined by the thickness of the thread and the weight of the fabric used for sewing in order to avoid  seam damages, low seam strength  and efficiency as well as sewing threads end breaks. 

A corresponding throat plate is, however, necessary because too large a throat plate can lead to lack of proper fabric control during the descent of the needle whereas too small a throat plate can lead to needle breakage when the needle is deflected ever so slightly. The effect of needle geometry on the sewability of knitted fabrics has also been investigated; it is found that the optimum needle could reduce sewing damage by as much as 35%. A size 12 ball-point needle running at 5.5 to 6.3 stitches/cm has been regarded as good all-round for knitted fabrics.10
A statistical analysis proved that increasing needle size from 12 to 16 leads to a reduction of seam tensile strength. Furthermore, it was found that increasing needle size from 12 to 16 leads to a reduction of the seam elongation by approximately 3.8%. A decreasing trend is detected assuring that the increase in the needle size leads to a reduction of the seam efficiency as shown in Figure (10) 

On the other hand, the needle bar has to be set properly depending on the feed timing. When the feed timing is advanced, the thread is pulled up with thread take up lever and caught between throat plate and material as well as thread tightness is deteriorated. When Feed Timing is delayed, bending of thread is decreased and upper thread tightness is improved. However, if feed timing is excessively delayed, needle wobbling occurs resulting in needle breakage, see figure (11).

All of these factors besides operator skill, threads size and quality and the environmental conditions would definitely affect the needle life. A prediction of needle life cannot be identified for each plant because it is impossible to know all individual factors. But a lot of sewing factories have the data available or are able to find the data for every sewing line or even for every sewing machine. This will give a good basis for quality improvement because if you have these data you will be able to implement a needle change policy in your factory as already a lot of the high quality manufacturers do. Figures (12) show examples of needles damages and breakages.

4)    Pressure foot and fed dogs:
The pressure foot is required to hold the fabric firmly during sewing. Improper control will displace the fabric from the normal sewing position and reduce the aesthetic performance of the seam due to puckering and staggered stitching.5
The drop-feed systems found on several types of sewing machines basically comprise three components: a presser foot, a throat plate and a feed dog.  Some of the problems in the sewing process rely on the interaction between the presser-foot and the feed dog.  In the case of elastic fabrics, a differential bottom feed is used, normally found on over edge, interlock and chain stitch sewing machines. Other feeding systems combine the drop-feed (simple or differential) with a feeding presser-foot and needle feed. These are used to work with heavier materials, and can virtually eliminate the problems of ply-shift. However, the problem of adequate presser-foot force adjustment is present in all of these systems. 13
Pressure foot Recess is made to decrease resistance and lift the thread smoothly when thread take up lever lifts upper thread. It is necessary to use a proper pressure foot in accordance with thickness of thread or stitch length. In case of large recess, the thread tightness is improved. However, if this pressure foot is used when stitch length is small or sewing light- weight materials, puckering may occur so the recommended feed pitch, in this case is approximately 4 mm. On the other hand, when feed pitch 3 mm; which means that the recess is small; defective thread tightness is about to occur. It is necessary to replace the pressure foot with one with large recess or to grind the recess to make it longer. If there is no recess at all, the seam puckering will be prevented by excessive thread tightness when sewing extra light weight materials, as per figure (13). 

To minimize seam puckering caused by uneven ply feeding, you should use 14
1)        The minimum presser foot pressure that will maintain uniform feeding.
2)        The feed dogs at their proper height and check for back-feeding.
3)        The correct presser foot and needle plate for the material and operation being sewn.
4)        Low friction presser foot: Teflon coated roller bearing, “feeding foot”, etc. Use an “anti puckering” needle plate with a retaining spring that holds back on the bottom ply to match the top ply.
5)        When the pressure of pressure foot is high feeding force is increased. However, uneven material feeding or feed dog mark on the cloth is likely to occur. Adjust the pressure to the lower level especially for the light weight material sewing or the like to such an extent that the material feeding is not difficult, as per figure (14). 
5)   Sewing Thread tension:
The tension regulator gives a proper tension to upper thread and lower thread among which can be adjusted as shown in Figure (15). Equal Tensions of lower and upper threads is greatly important for obtaining the optimum seam quality and aesthetics properties, see Figure (16). 

If a thread is sewn into the seam with excessive machine thread tension, the thread will try to recover or return to its original length. This will cause the seam to pucker immediately as the seam comes out from under the presser-foot as seen in Figure (17). 14
As seen in Figure (18), a decreasing trend is detected assuring that low sewing thread tension was associated with higher seam strength and high thread tension caused lower seam strength. It has been concluded that a decreasing trend is detected assuring that the increase in sewing thread tension lowers the seam elongation significantly. Moreover, it is noticed that the sewing thread tension has a negative impact on the seam efficiency. As the thread tension increases the seam efficiency decreases. The higher sewing thread tension associated with the lower seam.